The present invention relates to a semiconductor device having trench-type isolation regions, a floating gate, and a control gate. More particularly, the present invention relates to capacitive coupling between a floating gate and a control gate.
A nonvolatile semiconductor storage device is composed of nonvolatile memory cells. These nonvolatile memory cells each have a floating gate and a control gate. Potential of the floating gate is controlled by capacitive coupling between the floating gate and the control gate. As the capacitive coupling between the control gate and the floating gate increases, the voltage of the control gate can be lowered. In order to increase the capacitive coupling, the area where the control gate is proximal to the floating gate is increased as follows. The floating gate has a two-layered structure made of a polycrystalline silicon (Si) film in the first layer and polycrystalline silicon at a second layer. The polycrystalline silicon film of the first layer is formed by self-alignment with trench-type isolation regions. The polycrystalline silicon film of the second layer is formed onto the polycrystalline silicon film of the first layer and the trench type isolation regions. An insulation film is formed onto the polycrystalline silicon of the second layer. A control gate is formed onto this insulation film.
However, this cell structure requires a space, an isolation width “Wt” of the memory cell, for separating the polycrystalline silicon of the second layers. In addition, this cell structure requires a space, as an alignment margin, during lithography steps.
In this cell structure, in order to increase the capacitive coupling between the floating gate and the control gate, it is effective to increase the film thickness of the floating gate. When the film thickness of the floating gate is increased, capacitive coupling C2 in the floating gate between cells increases. When the capacitive coupling C2 increases, the cell threshold voltage is susceptible to modulation corresponding to the state of data in adjacent cells.